In a code division multiple access (CDMA) system, the vast majority of handoffs happen between cells on the same CDMA channel and use soft handoff procedures. On some occasions, the mobile stations need to perform a handoff between cells on different CDMA channels where such channels are at different radio frequencies (RF), often denoted as inter-frequency hard handoff. On some other occasions, the mobile stations need to perform a handoff from CDMA cells to cells that use a different Radio Access Technology (RAT), often denoted as inter-system or inter-RAT handoff, e.g. a CDMA to analog handover or a handoff between Wideband-CDMA (WCDMA) and Global System for Mobile Communications (GSM). Such situations are typically, but not limited to, either a handoff between different operators, a handoff between different RF channels allocated for capacity reasons, or a handoff between different signal modulation technologies.
Before effecting an inter-frequency or inter-system hard handoff, the mobile station is directed by the base station to tune to the new target frequency, measure the radio environment (e.g., pilot signal strength of the received signals, or RXLEV, RSSI and BSIC for GSM systems, etc.), and report the measurement back to the base station. Such a procedure is specified in TIA/EIA-95-B and in 3GPP TS 25.331v4.1.0 and greatly enhances the probability of success of an inter-frequency or inter-system handoff.
An essential requirement of the measurement on the target frequency, often referred to as “search excursion,” is to minimize the disruption of the current service on the originating frequency. The target frequency can be located on the same RAT as the originating frequency, or the target frequency can be located on a different RAT from the originating frequency. Handoffs to a second frequency in the same system or to a frequency in a different system could result in poor signal performance without adequate prior sampling. On the other hand, sampling for long periods of time may cause the signal at the originating frequency to be lost completely. The method described below permits the mobile station to minimize the search time and to limit the disruption of service.